Large area illumination sources based upon organic light emitting devices (OLED) have the potential to offer lighting customers a lighting source that provides energy savings and has a unique form factor. The potential energy efficiency gains are predicated on the assumption that the OLED has efficient electron to photon conversion (>0.9), operate at low voltage (<3.5V), exhibit high light extraction efficiencies (>0.7) and can be produced at a low enough cost to satisfy the general lighting market.
The optical performance of OLED devices is fundamentally limited by the difficulties associated with light extraction from large planar surfaces of moderate optical index (n=1.4-2.0) into the ambient. Using simple geometrical arguments, it is possible to show that the fraction of light coupled from an emitter embedded in an material of optical index nh into a medium of lower optical index n1 is proportional to (n1/nh)2. The higher the index of the OLED material, the lower the fraction of light extracted into the ambient (n=1). This single effect can reduce the total amount of light emitted by the OLED by over 50%. An additional complicating factor is that the light emission from an OLED occurs in the optical near field of the metallic electrodes; this leads to a pronounced quenching of the emission and a further reduction in efficiency. While it is possible to construct thicker devices, this tends to lead to power efficiency losses as the active layers of the OLED are increased in thickness. In the absence of corrective techniques, only about 20%-30% of the light generated within the device is actually emitted in to the ambient (n=1).
A number of schemes to increase the light output from OLED devices have been proposed, some involve texturing or patterning one or more interfaces or layers in the OLED to scatter light. Patterning the substrate allows for light that has been trapped in the substrate to be extracted, and increase the total light extraction, but does not affect the amount of light that has been lost due to coupling to the cathode of the device. One possible approach to reduce the losses due to the cathode is to increase the optical index of the substrate that supports the transparent anode material. For example, increasing the optical index of the substrate from n=1.5 to n=1.8 can increase the amount of light coupled into the substrate by a factor of 1.5. The problem then becomes the increased difficulty of extracting this light from the higher index substrate. It is thus necessary to compliment the use of a high index substrate with some degree of light scattering.
U.S. Pat. No. 6,777,871 (“Duggal et al.”) discloses an OLED device that includes an output coupler layer that includes a composite layer having a matrix material (e.g., an epoxy, silicone or other polymer materials) that is filled with scattering particles such as titania or zinc oxide. The particles are sized to result in effective scattering of light emitted from the OLED. Duggal et al. discloses an average particle size of 0.1 to 20 microns and preferably 1 to 10 microns. The matrix material may also include nanoparticles, such as titania or zinc oxide, having a size less than 100 nanometers to adjust the index of refraction of the composite outcoupling layer such that it is equal to or close to the index of refraction of the substrate. The output coupler layer is positioned on a transparent substrate, which supports an electrode and a light emitting organic layer.
Although Duggal et al. discloses the use of nanoparticles for light scattering effect, this reference teaches incorporating the nanoparticles in a matrix in conjunction with larger micron-sized particles. Using solely nanoparticles for light scattering effects would require a relatively large weight load of the nanoparticles in the outcouple layer, which would render the layer opaque and brittle.